Radio Frequency Identification (“RFID”) systems typically include RFID tags and RFID readers. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
Typically RFID systems use an RFID reader to interrogate one or more RFID tags. The reader transmits a Radio Frequency (“RF”) signal and performs the interrogation. The RF signal is an electromagnetic signal that is sensed by a tag that responds by transmitting back another RF signal. The tag generates the transmitted RF signal RF either originally, or by reflecting back a portion of the interrogating RF signal.
The RF signal that is reflected back by the tag may further encode data stored internally in the tag, such as a number. The data of the reflected signal is decoded by the reader, which thereby identifies, counts, or otherwise interacts with an item associated with tag. The decoded data could denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna system, a radio section, a power section, and frequently a logic section, a memory, or both. In early RFID tags, the power section was made up of an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or semi-active tags. However, advances in semiconductor technology have allowed RFID tags to be designed such that they can be powered solely by an RF signal received by the RFID tag. Such RFID tags do not include an energy storage device, and are thus called passive tags.
Harvesting sufficient power from the RF signal can be difficult since the voltage of the RF signal may be in the range of approximately 200 mV, and a typical supply voltage for circuits of the RFID tag is 1 V. If the amplitude of the signal is insufficient to operate the RFID tag, a power rectifier circuit may be used to increase the output DC voltage. The rectifiers in these systems must extract enough DC power from incident radiation to power the circuitry on the tag. However, rectification is difficult when the incident power levels are very low. Therefore, most rectifiers have an unresponsive dead zone at low voltage amplitudes. Small turn-on voltage of devices is one of the most important factors in rectifier design. This makes steep slope devices such as Tunnel or Tunneling Field Effect Transistors (“TFET”) attractive device options for this application.
Alternately, several compensation techniques have also been proposed recently to reduce the effective threshold voltage. However, these compensation techniques still need to deal with several issues, such as sensitivity to leakage current. Recent rectifier studies have focused on maximizing Power Conversion Efficiency (“PCE”) and output power, but not much on rectifier sensitivity. More emphasis has been placed on optimizing rectifier sensitivity with little emphasis on PCE and DC output power levels. PCE of a rectifier circuit is also affected by several parameters such as circuit topology, diode-device parameters, input RF signal frequency, amplitude, and output loading conditions.
Current passive RFID tags typically limit the communication range of an RFID to less than 3 meters. There are few rectifier topologies proposed so far which can perform efficiently at microwatts (1-100 μW) of available RF power with higher sensitivity and providing long range RF communication. Therefore, a design need is present for a passive RFID rectifier with high PCE, high sensitivity for long-range communication.